Design Procedure for Grate Inlets On-Grade

Use the following procedure for grate inlets on-grade:
  1. Compute the ponded width of flow (T). Use the outline provided in
    Gutter Ponding on Continuous Grades
    .
  2. Choose a grate type and size.
  3. Find the ratio of frontal flow to total gutter flow (E
    o
    ) for a straight cross-slope using Equation 10-5. No depression is applied to a grate on-grade inlet.
  4. Find the ratio of frontal flow intercepted to total frontal flow, R
    f
    , using Equations 10-22, 10-23, and 10-24.
    If v > v
    o
    , use Equation 10-22.
    EquationObject299355
    Equation 10-22.
    If v < v
    o
    , use Equation 10-23.
    EquationObject300356
    Equation 10-23.
    where:
    • R
      f
       = ratio of frontal flow intercepted to total frontal flow
    • K
      u
       = 0.09 for English measurement or 0.295 for metric
    • v = approach velocity of flow in gutter (ft./s or m/s)
    • v
      o
       = minimum velocity that will cause splash over grate (ft./s or m/s)
    For triangular sections, calculate the approach velocity of flow in gutter (v) using Equation 10-24.
    EquationObject301357
    Equation 10-24.
    Otherwise, compute the section flow area of flow (A) and calculate the velocity using Equation 10-25:
    EquationObject302358
    Equation 10-25.
    where:
    • v
      o
       = splash-over velocity (ft./s or m/s)
    • L = length of grate (ft. or m)
    Calculate the minimum velocity (v
    o
    ) that will cause splash over the grate using the appropriate equation in Table 10-2 below.
    Table 10-2. Splash-Over Velocity Calculation Equations (English)
    Grate Configuration
    Typical Bar Spacing (in.)
    Splash-over Velocity Equation
    Parallel Bars
    2
    Parallel Bars
    1.2
    Parallel bars w/ transverse rods
    2 parallel/4 transverse
    Reticuline
    n/a
  5. Find the ratio of side flow intercepted to total side flow, R
    s
    .
    EquationObject303363
    Equation 10-26.
    where:
    • R
      S
       = ratio of side flow intercepted to total flow
    • z
       = 0.15 for English measurement or 0.083 for metric
    • S
      x
       = transverse slope
    • v
       = approach velocity of flow in gutter (ft./s or m/s)
    • L
       = length of grate (ft. or m).
  6. Determine the efficiency of grate, E
    f
    . Use Equation 10-27.
    EquationObject304364
    Equation 10-27.
  7. Calculate the interception capacity of the grate, Q
    i
    . Use Equation 10-28. If the interception capacity is greater than the design discharge, skip step 8.
    EquationObject305365
    Equation 10-28.
  8. Determine the bypass flow (CO) using Equation 10-29.
    EquationObject306366
    Equation 10-29.
  9. Depending on the bypass flow, select a larger or smaller inlet as needed. If the bypass flow is excessive, select a larger configuration of inlet and return to step 3. If the interception capacity far exceeds the design discharge, consider using a smaller inlet and return to step 3.